1. Related Applications
This application is related to copending applications entitled "Solid State Formation of Sapphire Using a Localized Energy Source", Ser. No. 08/064,386 (LD 10498), filed May 21, 1993; "Solid State Thermal Conversion of Polycrystalline Alumina to Sapphire Using a Seed Crystal", Ser. No. 08/126,628, filed Sep. 24, 1993; and "Conversion of Polycrystalline Material to Single Crystal Material Using Bodies Having a Selected Surface Topography", Ser. No. 08/126,830, filed Sep. 24, 1993, the latter two applications filed concurrently herewith, all of which are assigned to the assignee of the present invention.
Another related copending application is Ser. No. 08/195,187 filed Feb. 14, 1994 entitled "Conversion of Doped Polycrystalline Material to Single Crystal Material."
2. Field of the Invention
This invention relates to a solid state process for bulk conversion of a polycrystalline ceramic body to a single crystal body by heating the polycrystalline body to temperatures above one-half the melting temperature of the material but below the melting temperature of the material. More particularly, this invention relates to a solid state process for the bulk conversion of polycrystalline alumina (PCA) to a single crystal alumina (sapphire). The solid state conversion of PCA to sapphire is accomplished by heating a PCA body containing less than 100 wppm of magnesia to a temperature above 1100.degree. C. but below 2050.degree. C., the melting point of alumina. No melting of the PCA body is necessary to obtain the conversion.
3. Background of the Invention
The manufacture of Lucalox.RTM. alumina, a type of polycrystalline alumina (PCA), and its use for high pressure sodium arc discharge lamps (hereinafter "HPS lamps") is well known and old to those skilled in the art. U.S. Pat. Nos. 3,026,210; 4,150,317 and 4,285,732, respectively, to Coble, Laska et al and Charles et al, disclose the production of a high density PCA body having improved visible light transmittance using relatively pure alumina powder and small amounts of magnesia. U.S. Pat. No. 4,285,732 further teaches adding zirconia and hafnia to the magnesia-doped alumina to reduce the chances of precipitating a second phase and exaggerated or run away grain growth during sintering. PCA bodies useful for HPS lamps have been made according to the process in these patents having an average grain size ranging between 15 microns to 100 microns. Two major drawbacks associated with the use of PCA arc tubes for HPS lamps are firstly that they are light translucent as opposed to light transparent and secondly, that the sodium in the arc reacts with the alumina at the PCA grain boundaries to form sodium aluminate, which shortens lamp life. HPS lamps are being designed for increased sodium partial pressure within the PCA arc tube to improve the color rendition and provide a whiter emitted light. However, higher sodium pressure further shortens lamp life due to sodium loss from the arc chamber. Progressive sodium loss results in a rise in the lamp operating voltage, a decrease of both correlated color temperature and color rendering index, and a color shift from white to pink. Also, the sodium which migrates through the arc chamber wall deposits on the inside wall of the evacuated outer lamp envelope causing a brownish stain on the envelope which, in turn, further reduces the light output of the lamp. These problems are substantially reduced with sapphire (i.e., single crystal alumina) arc tubes.
Sapphire arc tubes useful as the arc chamber for HPS lamps have been made by number of processes, including a modified Czochralski process known as the edge-defined, film-fed growth (EFG) process developed by Tyco Laboratories, Inc. This process uses a seed crystal and a die on the surface of molten alumina in which a hollow tube is continuously pulled out of the melt through a die. This process is expensive and slow. Another process used to produce single crystal alumina (sapphire) is called the floating zone process. In this process, a PCA rod is introduced at a predetermined velocity into a heating zone wherein one or more lasers, or other concentrated heat source, are focused on the rod to melt the alumina, forming a melt zone. A sapphire fiber is continuously drawn from the melt zone at desired velocity and the feed rod is moved simultaneously, but at a slower rate so that the process is a continuous one. This process is used primarily to produce sapphire fibers and does not readily lend itself to production of single crystal alumina tubing, although its use for such is disclosed in U.S. Pat. No. 3,943,324.
Published Japanese Patent Application 62-28118 of H. Yoshida et al, discloses that sapphire can be synthesized via a solid state process by inducing a magnesia concentration gradient along the length of a PCA body to ensure grain growth is initiated from a single point on the PCA body during the heat treatment to produce a single crystal body. This magnesia gradient can be produced in the PCA body by doping the green body with magnesia in such a way that there is a magnesia gradient in the PCA body or by using a temperature gradient to create the magnesia concentration gradient, or by machining a thin section on the green body. Key to the Yoshida et al process is that the growth of the single crystal initiates from a single location in the polycrystalline body. The Yoshida et al disclosure, taken at face value, appears not to be workable. In particular, Yoshida et al require only the equivalent of 90 wppm (weight parts per million) magnesia in their alumina starting body. Yet, in order to realize a dense PCA structure, at least about 300 wppm of magnesia is required. (Note "Alumina: Sintering and Optical Properties", J. G. J. Peelen PhD Thesis, Technical University of Eindhoven, Netherlands, May 1977.) Typical conversion processes, such as those used to manufacture Lucalox.RTM., have 550-750 wppm magnesia in the alumina starting body to ensure full density is achieved. At 90 wppm of magnesia, a dense, pore-free structure, specified by Yoshida et al as their starting material, cannot be achieved.